Resonant illumination driver in an optical distance measurement system

ABSTRACT

An optical transmitting system for distance measuring includes a modulation signal generator, a light source, and an illumination driver coupled to the modulation signal generator and the light source. The modulation signal generator is configured to generate a modulation signal. The light source is configured to generate an optical waveform with amplitude modulation corresponding with the modulation signal. The illumination driver is configured to drive the light source. The illumination driver includes a switch and a switch driver. The switch is configured to switch between an on state and an off state to drive the light source. The switch driver is configured to drive the switch between the on and off states. The switch driver includes a first inductor and a capacitor in series with the first inductor and the switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Indian Provisional PatentApplication No. 201641026036, filed Jul. 29, 2016, titled “ResonantIllumination Driver for Achieving Higher Electrical to OpticalEfficiency in 3D TOF Camera Illumination Design,” which is herebyincorporated herein by reference in its entirety.

BACKGROUND

Optical time of flight (TOF) systems generally use optical light signalsto measure distances to objects based on the time of flight of the lightsignal to the target object and back to the system. For example,three-dimensional (3D) TOF camera systems work by measuring the distanceto a target object by reflecting light off of one or more targets andanalyzing the reflected light. More specifically, 3D TOF camera systemstypically determine a time of flight (TOF) for the light pulse to travelfrom the light source (e.g., a laser or light emitting diode (LED)) to atarget object and return by analyzing the phase shift between thereflected light signal and the transmitted light signal. The distance tothe target object then may be determined. An entire scene is capturedwith each transmitted light pulse. These systems may be used in manyapplications including: geography, geology, geomorphology, seismology,transport, human-machine interfaces, machine vision, and remote sensing.For example, in transportation, automobiles may include 3D TOF camerasystems to monitor the distance between the vehicle and other objects(e.g., another vehicle). The vehicle may utilize the distance determinedby the 3D TOF camera system to, for example, determine whether the otherobject, such as another vehicle, is too close, and automatically applybraking.

SUMMARY

In accordance with at least one embodiment of the disclosure, an opticaltransmitting system for distance measuring includes a modulation signalgenerator, a light source, and an illumination driver coupled to themodulation signal generator and the light source. The modulation signalgenerator is configured to generate a modulation signal. The lightsource is configured to generate an optical waveform with amplitudemodulation corresponding with the modulation signal. The illuminationdriver is configured to drive the light source. The illumination driverincludes a switch and a switch driver. The switch is configured toswitch between an on state and an off state to drive the light source.The switch driver is configured to drive the switch between the on andoff states. The switch driver includes a first inductor and a capacitorin series with the first inductor and the switch.

Another illustrative embodiment is a resonant illumination driver thatincludes a first inductor, a second inductor in series with the firstinductor, a capacitor in series with the first and second inductors, anda power transistor in series with the capacitor. The first inductor isconfigured to receive a drive current. The power transistor isconfigured to switch between an on state and an off state to drive alight source.

Yet another illustrative embodiment is a 3D TOF camera that includes atransmitter and a receiver. The transmitter is configured to generate anoptical waveform with amplitude modulation corresponding with afrequency of a generated modulation signal. The transmitter includes anillumination driver configured to drive a light source that generatesthe optical waveform. The illumination driver includes a switch and aswitch driver. The switch is configured to switch between an on stateand an off state to drive the light source. The switch driver isconfigured to drive the switch between the on and off states. The switchdriver includes a first inductor and a second inductor in a splitconfiguration with the first inductor and a capacitor in series with thefirst and second inductors and the switch. The receiver is configured toreceive the optical waveform reflected off of a target object anddetermine a distance to the target object based on a TOF from thetransmitter to the target object and back to the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an illustrative optical time of flight system in accordancewith various examples;

FIG. 2 shows an illustrative transmitter for an optical time of flightsystem in accordance with various examples;

FIG. 3 shows an illustrative illumination driver for a transmitter foran optical time of flight system in accordance with various examples;

FIG. 4 shows multiple illustrative voltage versus time graphs at variousnodes in an illumination driver for a transmitter for an optical time offlight system in accordance with various examples;

FIG. 5A shows an illustrative receiver for an optical time of flightsystem in accordance with various examples; and

FIG. 5B shows an illustrative receiver for an optical time of flightsystem in accordance with various examples.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, companies may refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function. In the following discussion and in the claims,the terms “including” and “comprising” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to . . . ” Also, the term “couple” or “couples” is intended tomean either an indirect or direct connection. Thus, if a first devicecouples to a second device, that connection may be through a directconnection, or through an indirect connection via other devices andconnections. The recitation “based on” is intended to mean “based atleast in part on.” Therefore, if X is based on Y, X may be based on Yand any number of other factors.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of thedisclosure. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

Optical TOF systems, such as 3D TOF cameras, point Light Detection andRanging (LiDAR, LIDAR, lidar, LADAR), and scanning LIDAR, determinedistances to various target objects utilizing the TOF of an opticalsignal (e.g., a light signal) to the target object and its reflectionoff a target object back to the TOF system (return signal). Thesesystems can be used in many applications including: geography, geology,geomorphology, seismology, transport, and remote sensing. For example,in transportation, automobiles can include 3D cameras to monitor thedistance between the vehicle and other objects (e.g., another vehicle).The vehicle can utilize the distance determined by the 3D camera to, forexample, determine whether the other object, such as another vehicle, istoo close, and automatically apply braking.

The optical signals are generated by a light source (e.g., a laserdiode, light emitting diode, etc.) driven by an illumination driver. Inorder to generate amplitude modulated optical signals, which areutilized in many optical TOF systems, the illumination driver hardswitches one or more power switches (e.g., a powermetal-oxide-semiconductor field effect transistor (MOSFET)) at amodulation signal frequency. The frequency of switching is typically inthe tens to hundreds of MHz. At these frequencies, the gate capacitancesof the power switches, which typically have a drain-to-source resistancewhen closed of a few milliohms, are relatively high and demand arelatively high amount of power (e.g., approximately 1 W). Conventionalillumination drivers either directly drive the gate of the power switchor use a simple resonant circuit that uses the gate capacitance of thepower switch and a single inductor. However, due to the high frequencyof switching, directly driven drivers have high gate driving losses inthe tens of MHz. Additionally, it is difficult to drive lowdrain-to-source resistance when closed switches at a tens to hundreds ofMHz switching frequency. For simple resonant circuits, the gatecapacitance is variable at each switch. Thus, it is difficult to utilizea single design across different devices. Furthermore, the gatecapacitance changes as temperature changes. Thus, the switchingfrequency generated by the illumination driver can deviate from thedesired frequency. Thus, there is a need for an illumination driver thatreliably provides switching to a power switch at a frequency in the tensto hundreds of MHz to drive a light source in an optical TOF system.

In accordance with various examples, an optical TOF system is providedwith an illumination driver that includes a resonant circuit with aresonant frequency that is unaffected by the gate capacitance of thepower switch. The resonant circuit includes, in an embodiment, twoinductors in a split configuration in series with a relatively lowcapacitance capacitor and the gate of the power switch. The resonantfrequency of the circuit is determined based on the value of theinductance of the two inductors and the capacitance of the capacitor. Byadding the series capacitor, the resonant frequency of the resonantcircuit is unaffected by the gate capacitance of the power switchbecause the effective capacitance of the resonant circuit is equal tothe capacitance of the series capacitor which does not vary acrossdevices and is temperature invariant. Hence, the resonant frequency ofthe illumination driver can be reliably generated without the need tocompensate for temperature and/or device variations.

To provide the necessary voltage to drive the gate of the power switchwith a relatively low capacitance series capacitor while ensuringcompatibility with complementary metal-oxide semiconductor (CMOS) designlimitations, the combination of the split inductors can generate highervoltages (e.g., up to 50V) at the input of the series capacitor.Therefore, the illumination driver can generate a voltage that can openand close the power switch even in the presence of the relatively lowcapacitance series capacitor.

FIG. 1 shows an illustrative optical TOF system 100 in accordance withvarious examples. In some embodiments, the optical TOF system 100 is a3D TOF camera. However, the optical TOF system 100 can be any type ofoptical TOF system (e.g., point LIDAR, scanning LIDAR, etc.). Theoptical TOF system 100 includes a transmitter 102, receiver 110, andcontroller 112. The transmitter 102 is configured, by the controller112, to generate one or more optical waveforms 152. The controller 112can be implemented as a processor (e.g., a microcontroller, ageneral-purpose processor, etc.) that executes instructions retrievedfrom a storage device, or as dedicated hardware circuitry. In someembodiments, the optical waveform 152 is a single tone (e.g., acontinuous wave) with amplitude modulation (e.g., a continuous amplitudemodulated waveform).

The transmitter 102 is also configured, in an embodiment, to direct theoptical waveform 152 toward the field of view (FOV) 106. In someembodiments, the transmitter 102 directs the optical waveform 152 towardthe FOV 106 by directing the optical waveform 152 directly to the FOV106. In other embodiments, the transmitter 102 directs the opticalwaveform 152 toward the FOV 106 by directing the optical waveform to abeam steering device (not shown) which then directs the optical waveform152 to the FOV 106. In such embodiments, the beam steering devicereceives the optical waveform 152 from the transmitter 102 and steersthe optical waveform 152 to the FOV 106. Thus, the transmitter 102 candirect the optical waveform 152 directly to the target object 106 or candirect the optical waveforms 152 to a beam steering device which directsthe optical waveform 152 to the FOV 106.

The optical waveform 152 (or optical waveforms 152) reflects off of anyobjects located within the FOV 106 (i.e., target objects) and returnstoward the receiver 110 as reflected optical waveform 162. The reflectedoptical waveform 162 is then received by the receiver 110. In someembodiments, an additional beam steering device (not shown) steers thereflected optical waveform 162 to the receiver 110. In some embodiments,the receiver 110 receives the reflected optical waveform 162 directlyfrom the target object 106.

The receiver 110 is configured to receive the reflected optical waveform162 and determine the distance to the target objects within FOV 106based on the TOF from the transmitter 102 to the target object 106 andback to the receiver 110. For example, the speed of light is known, sothe distance to the target objects is determined and/or estimated usingthe TOF. That is, the distance is estimated as

$d = \frac{c*{TOF}}{2}$

where d is the distance to the target object, c is the speed of light,and TOF is the time of flight. The speed of light times the TOF ishalved to account for the travel of the light pulse to, and from, thetarget object.

In some embodiments, the receiver 110, in addition to receiving thereflected optical waveform 162 reflected off of the target object 106,is also configured to receive the optical waveform 152, or a portion ofthe optical waveform 152, directly from the transmitter 102. Thereceiver 110, in an embodiment, is configured to convert the two opticalsignals into electrical signals, a received signal corresponding to thereflected optical waveform 162 and a reference signal corresponding tothe optical waveform 152 received directly from the transmitter 102. Thereceiver 110 then, in an embodiment, performs a correlation functionusing the reference signal and the received signal. A peak in thecorrelation function corresponds to the time delay of the receivedreflected optical waveform 162 (i.e., the TOF). The distance then can beestimated using the formula discussed above. In other embodiments, afast Fourier transform (FFT) can be performed on the received signal. Aphase of the tone then is used to estimate the delay (i.e., TOF) in thereceived signal. The distance then can be estimated using the formuladiscussed above. In yet other embodiments, the in-phase (I) component isdetermined by correlating the received reflected optical waveform 162with the transmitted optical waveform 152 received directly from thetransmitter 102, and the quadrature (Q) component is determined bycorrelating the received reflected optical waveform 162 with a 90 degreephase shifted version of the transmitted optical waveform 152 receiveddirectly from the transmitter 102. The I/Q integrated charges are usedto estimate the phase shift between the optical waveform 152 receiveddirectly from the transmitter 102 and the received reflected opticalwaveform 162. The distance then can be estimated using the formuladiscussed above.

FIG. 2 shows an illustrative transmitter 102 for optical TOF system 100in accordance with various examples. The transmitter 102, in anembodiment, includes a modulation signal generator 202, an illuminationdriver 206, a light source 208, and an optics device 210. The modulationsignal generator 202 is configured to generate a modulation signal(e.g., a modulation reference signal) and continuous waveforms using themodulation signal. For example, in some embodiments, the modulationsignal generator 202 is configured to generate a single tone (i.e.continuous wave) modulation signal. The amplitude of the carrier signal,which in an embodiment is also generated by the modulation signalgenerator 202, is modulated with the modulation signal to generate amodulated carrier signal.

The illumination driver 206 generates a driving signal (regulates thecurrent) to drive one or more optical transmitters, such as light source208, so that the optical transmitter generates optical transmissionsignal 152 that corresponds with the modulated carrier signal generatedby the modulation signal generator 202. In other words, the modulationsignal modulates the intensity of the light transmitted by light source208 during the pulse with the illumination driver 206 providing thedriving current to the light source 208. The amplitude of the modulatedcarrier signal, and thus, the amplitude of the optical waveform 152depends on and thus, corresponds with, the frequency of the modulationsignal. While light source 208 is shown in FIG. 2 as a laser diode, anytype of optical signal generator (e.g., a light emitting diode (LED))can be utilized to generate the optical waveform 152. The optical device210, which, in an embodiment is one or more lenses, is configured todirect (e.g., focus) the optical waveform 152 (e.g., the modulated lightsignal) toward the FOV 106.

FIG. 3 shows an illustrative illumination driver 206 for transmitter 102for optical TOF system 100 in accordance with various examples. Theillumination driver 206, in an embodiment, includes a switch driver 302and a switch 304. The switch driver 302 is configured to drive the gateof switch 304 with a voltage and/or current signal to open and closeswitch 304. In other words, the switch driver 302 generates a voltageand/or current that causes the switch 304 to enter an on state (theswitch closes) and then a voltage and/or current that causes the switch304 to enter an off state (the switch opens). More particularly, in anembodiment, the switch driver 302 generates a voltage that is greaterthan the threshold voltage of the switch 304 thereby closing the switch304 (causing the switch 304 to enter the on state) alternating withgenerating a voltage that is less than the threshold voltage of theswitch 304 thereby opening the switch 304 (causing the switch 304 toenter the off state).

In some embodiments, the switch 304 is configured to switch between theon state and the off state at a frequency in the tens of MHz up to 200MHz. The switching of the switch 304, along with, in some embodiments,additional similar switches provides the drive current to drive thelight source 208. More particularly, the amplitude modulation of opticalsignal 152 is determined by the switching frequency of the switch 304which, in turn, corresponds with the carrier modulated signal generatedby the modulation signal generator 202. In some embodiments, switch 302is a power transistor. More particularly, the switch 304, in someembodiments, is a power n-type metal oxide semiconductor (NMOS)field-effect transistor. However, switch 304 can be any type ofelectrical switch, such as a p-type metal oxide semiconductor (PMOS)field-effect transistor, a binary junction transistor (BJT), etc.

The switch driver 302 includes, in an embodiment, a level shifter 306, aresistor 308, inductors 310-312, and capacitor 314. Level shifter 306 isconfigured to shift the low and high levels of a digital signal outputfrom one part of a system to different low and high levels required byanother part of the system. For example, one part of an electricalsystem may operate as 1.5V CMOS, where LOW (i.e., 0) is represented by avoltage between 0 and 0.1V and HIGH (i.e., 1) is represented by avoltage between 1.4V and 1.5V, while the illumination driver 208operates as 5V CMOS, where LOW is represented by a voltage between 0 and0.1V and HIGH is represented by a voltage between 4.9V and 5.0V. Inother words, level shifter 306 is configured to shift the HIGH signalfrom being represented by a voltage between 1.4V and 1.5V to beingrepresented by a voltage between 4.9V and 5.0V. Thus, the level shifter306 acts as a current and/or voltage source for the switch driver 302.

The inductors 310-312 and capacitor 314 are configured as a resonantcircuit with a resonant frequency that is approximately (i.e., plus orminus 10%) equal to the frequency of the modulation signal. Thus, theswitch 304 switches between the on state and the off state at themodulation signal frequency, creating the desired amplitude modulationin the optical waveform 152. More particularly, the inductors 310 and312 are in a split configuration. The capacitor 314 is in series withthe inductors 310-312. For example, the inductor 310 includes a firstend which is connected to a first end of the inductor 312 and a secondend which is connected to the capacitor 314. In addition to beingconnected to inductor 310 at its first end, inductor 312 is connected ata second end to the source of switch 304. In addition to being connectedto the second end of inductor 310, capacitor 314 is connected to thegate of switch 304.

In some embodiments, the capacitance of the capacitor 314 is less thanthe gate capacitance of the switch 304. For example, the capacitance ofthe capacitor 314 can be on the order of 50 pF while the gatecapacitance of the switch 304 generally ranges from hundreds of pF toapproximately 1 nF. Thus, the capacitance of the capacitor 314 is, in anembodiment, at least less than 10 times less than the gate capacitanceof the switch 304. Hence, the combination of the capacitor 314 with thegate capacitance of the switch 304 acts as a capacitive divider withmost of the voltage across the capacitor 314. Thus, the effectivecapacitance of the resonant circuit (switch driver 302) is equal to thecapacitance of the capacitor 314.

As discussed above, conventional simple resonant circuit switch driversuse only the gate capacitance of the power switch to create the resonantcircuit. However, the gate capacitance of the power switch varies acrossdevices and even within the same switch as temperatures change. Thus, itis difficult to generate/maintain the desired resonant frequency, andthus, the desired switching frequency. By adding the series capacitor314, the resonant frequency of the switch driver 302 is unaffected bythe gate capacitance of the switch 304 because the effective capacitanceof the resonant circuit is equal to the capacitance of the capacitor 314which does not vary across devices and is temperature invariant. Hence,the resonant frequency of the switch driver 302 can be reliablygenerated without the need to compensate for temperature and/or devicevariations.

The gate threshold voltage of the switch 304 is, in an embodiment,approximately 5V. Because the series capacitor 314 is included in theswitch driver 302, a relatively high voltage (e.g., 20V to 50V) isapplied to the capacitor 314 to drive the gate of switch 304 atapproximately 5V. However, output voltages in CMOS systems are typicallylimited to a maximum of 5V. Therefore, given the limitations of theavailable CMOS drive output voltages, the combination of inductors 310and 312 ensures sufficient voltage at the gate of switch 304 whileensuring compatibility with the excitation circuit. In an embodiment,the inductance of inductor 312 is less than the inductance of inductor310. Because the inductors 310-312 are in series, the combination ofinductors 310-312 can generate higher voltages (e.g., up to 50V) at theinput of capacitor 314. Therefore, the switch driver 302 can generatethe 5V to switch the switch 304 between the on and off states.Additionally, in some embodiments, the direct current (DC) voltage atthe gate of switch 304 is maintained by a high impedance voltage source(not shown) and a capacitor (not shown) is added in series with theswitch driver 302 to avoid DC losses to the switch driver 302.

The illumination driver 206 has several advantages over conventional LCresonant drivers. For example, the series capacitance added by capacitor314 may ensure that there is negligible or no frequency dependency onthe gate capacitance of switch 304. Thus, there is negligible or nofrequency variations from board to board. Additionally, without thecapacitor 314, the inductance values for a 50 MHz switching frequencywould be approximately 10 nH-20 nH which is low enough to make theillumination driver 206 susceptible to board and/or package parasiticinductance. However, with the inductors 310-312 and series capacitor314, the inductance is, in some embodiments, 100 nH-200 nH which is highenough to make the illumination driver not susceptible to board and/orpackage parasitic inductance. Furthermore, a single design can beutilized for many different drivers instead of each driver having to bedesigned for the specific switch in that driver.

Moreover, the illumination driver 206 has several advantages overconventional direct switch drivers. For example, there are lower gatedriving losses in the illumination driver 206. Additionally, it ispossible to drive lower drain-to-source resistance switches which havehigher gate capacitance with illumination driver 206. Furthermore, thepower levels required by illumination driver 206 are much lower (e.g.,150 mW) than the conventional direct switch drivers (e.g., 1 W).

FIG. 4 shows multiple illustrative voltage versus time graphs 402-408 atvarious nodes 322-328, respectively, in illumination driver 206 fortransmitter 102 for optical TOF system 100 in accordance with variousexamples. The graph 402 shows an example voltage versus time graph forthe voltage level at the node 322 in FIG. 3 (labelled as V₃₂₂). As shownin graph 402, the voltage is, in an embodiment, level shifted by levelshifter 306 to a 5V peak square waveform.

As shown in example graph 404, the voltage at node 324 (the node betweenthe split inductors 310-312 and labelled as V₃₂₄), is a sine waveformcreated from the square waveform V₃₂₂ shown in graph 402. The waveformV₃₂₄ peaks at 1.5V and oscillates between −1.5V and 1.5V. As shown inexample graph 406, the voltage at node 326 (the node between theinductor 310 and capacitor 314 and labelled as V₃₂₆), is a sine waveformcreated from the waveform V₃₂₄ shown in graph 404. The waveform V₃₂₆peaks at 20V and oscillates between −20V and 20V. Thus, the inductors310-312, as discussed above, are able to generate a relatively largevoltage at the capacitor 314. As shown in example graph 408, the voltageat node 328 (the node between the capacitor 314 and the gate of theswitch 304 and labelled as V₃₂₈), is a sine waveform created from thewaveform V₃₂₆ shown in graph 406. The waveform V₃₂₈ peaks at 5V andoscillates between 0V and 5V. Thus, the inductors 310-312 and capacitor314, as discussed above, are able to generate a gate drive voltage toopen and close switch 304 at the resonant frequency which, in anembodiment, is approximately equal to the modulation signal frequency.In this way, the illumination driver 206 drives the light source 208 ata switching frequency which causes the light source 208 to generate theoptical waveform 152 with the desired amplitude modulation.

FIG. 5A shows an illustrative optical receiver 110 for optical TOFsystem 100 in accordance with various examples. The receiver 110includes, in an embodiment, an optics device 510 (e.g., a lens), twophotodiodes 502 and 512, two trans-impedance amplifiers (TIAs) 504 and514, two analog-to-digital converters (ADCs) 506 and 516, and a receiverprocessor 508. As discussed above, in an embodiment, the reflectedoptical waveform 162 is received by the receiver 110 after reflectingoff of target objects within the FOV 106. The optics device 510, in anembodiment, receives the reflected optical waveform 162. The opticsdevice 510 directs (e.g., focuses) the reflected optical waveform 162 tothe photodiode 512. The photodiode 512 is configured to receive thereflected optical waveform 162 and convert the reflected opticalwaveform 162 into a current received signal 552 (a current that isproportional to the intensity of the received reflected light). TIA 514is configured to receive the current received signal 552 and convert thecurrent received signal 552 into a voltage signal, designated as voltagereceived signal 554 that corresponds with the current received signal552. ADC 516 is configured to receive the voltage received signal 554and convert the voltage received signal 554 from an analog signal into acorresponding digital signal, designated as digital received signal 556.Additionally, in some embodiments, the current received signal 552 isfiltered (e.g., band pass filtered) prior to being received by the TIA514 and/or the voltage received signal 554 is filtered prior to beingreceived by the ADC 516. In some embodiments, the voltage receivedsignal 554 is received by a time to digital converter (TDC) (not shown)to provide a digital representation of the time that the voltagereceived signal 554 is received.

Photodiode 502, in an embodiment, receives the optical waveform 152, ora portion of the optical waveform 152, directly from the transmitter 102and converts the optical waveform 152 into a current reference signal562 (a current that is proportional to the intensity of the receivedlight directly from transmitter 102). TIA 504 is configured to receivethe current reference signal 562 and convert the current referencesignal 562 into a voltage signal, designated as voltage reference signal564 that corresponds with the current reference signal 562. ADC 506 isconfigured to receive the voltage reference signal 564 and convert thevoltage reference signal 564 from an analog signal into a correspondingdigital signal, designated as digital reference signal 566.Additionally, in some embodiments, the current reference signal 562 isfiltered (e.g., band pass filtered) prior to being received by the TIA504 and/or the voltage reference signal 564 is filtered prior to beingreceived by the ADC 506. In some embodiments, the voltage referencesignal 564 is received by a TDC (not shown) to provide a digitalrepresentation of the time that the voltage reference signal 564 isreceived.

The processor 508 is any type of processor, controller, microcontroller,and/or microprocessor with an architecture optimized for processing thedigital received signal 556 and/or the digital reference signal 566. Forexample, the processor 508 can be a digital signal processor (DSP), acentral processing unit (CPU), a reduced instruction set computing(RISC) core such as an advanced RISC machine (ARM) core, a mixed signalprocessor (MSP), etc. In some embodiments, the processor 508 is a partof the controller 112. The processor 508, in an embodiment, acts todemodulate the digital received signal 556 and the digital referencesignal 566 based on the modulation signal generated by the modulationsignal generator 202. In some embodiments, the processor 508 alsoreceives the digital representation of the times that the voltagereceived signal 556 and the digital reference signal 566 were received.The processor 508 then determines, in an embodiment, the distance totarget objects within the FOV 106 by, as discussed above, performing acorrelation function using the reference signal and the received signal.A peak in the correlation function corresponds to the time delay of eachreceived reflected optical waveform 162 (i.e., the TOF). The distance tothe target object 106 can be estimated using the formula discussedabove. In other embodiments, an FFT is performed on the received digitalsignal 556. A phase of the tone then is used to estimate the delay(i.e., TOF) in the received signals. The distance then can be estimatedusing the formula discussed above.

FIG. 5B shows another illustrative receiver 102 for optical TOF system100 in accordance with various examples. The example receiver 102 shownin FIG. 5B is an I/Q receiver. The receiver 102 includes a photodiode582, two switches 586 and 596, and two capacitors 584 and 594. Thephotodiode 582 receives the reflected optical waveform 162 from thetarget object 106 and converts the reflected optical waveform 162 into acurrent which is proportional to the amount of light being received bythe photodiode 582. Because the amplitude of the reflected opticalwaveform 162 is being modulated, this current is representative of theamplitude modulation of the reflected optical waveform 162 and thus, thetransmitted optical waveform 152. By closing switch 586, this current isintegrated on capacitor 584. The capacitor 584 integrates the currentand collects charge. The switch 586, in an embodiment, is opened andclosed using the modulation signal generated by the modulation signalgenerator 202. This correlates the transmitted optical waveform 152 withthe received reflected optical waveform 162. Thus, the charge on thecapacitor 584 is effectively the correlation of the received reflectedoptical waveform 162 with the modulation signal. The switch 596 isclosed using an orthogonal (90 degree phase shifted) version of themodulation signal (the quadrature phase). Thus, the capacitor 594integrates the current and collects charge based on this quadraturephase of the modulation signal. This correlates the quadrature phase ofthe transmitted optical waveform 152 with the received reflected opticalwaveform 162. The charge on capacitors 584 and 594 represent the resultof the correlation of the received reflected optical signal 162 withrespect to the transmitted optical signal 152. This is then used tocalculate the distance to the target object 106 as discussed above.

In other words, the I/Q receiver 110 shown in FIG. 5B converts thereceived reflected optical waveform 162 into an electrical receivedsignal (e.g., a current). This electrical received signal is correlatedwith the modulated carrier signal generated by the modulation signalgenerator 202 (corresponding to the transmitted optical signal 152) togenerate an I component. The electrical received signal is alsocorrelated with a 90 degree phase shifted version of the modulatedcarrier signal to generate a Q component. The phase shift between theelectrical received signal and the modulated carrier signal is estimatedbased on the I component and the Q component. This phase shift isconverted into the distance to the target object as discussed above.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present disclosure. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. An optical transmitting system for distancemeasuring, comprising: a modulation signal generator configured togenerate a modulation signal; a light source configured to generate anoptical waveform with amplitude modulation corresponding with themodulation signal; and an illumination driver coupled to the modulationsignal generator and the light source, the illumination driverconfigured to drive the light source, the illumination driver including:a switch configured to switch between an on state and an off state todrive the light source; and a switch driver configured to drive theswitch between the on and off states, the switch driver including afirst inductor and a capacitor in series with the first inductor and theswitch.
 2. The optical transmitting system of claim 1, wherein acapacitance of the capacitor is less than a gate capacitance of theswitch.
 3. The optical transmitting system of claim 2, wherein thecapacitance of the capacitor is at least ten times less than the gatecapacitance of the switch.
 4. The optical transmitting system of claim1, wherein: a first end of the first inductor is connected to a firstend of a second inductor and a second end of the first inductor isconnected to the capacitor; a second end of the second inductor isconnected to a source of the switch; and the capacitor is connected to agate of the switch.
 5. The optical transmitting system of claim 4,wherein an inductance of the second inductor is less than an inductanceof the first inductor.
 6. The optical transmitting system of claim 4,wherein the first inductor and second inductors and the capacitor areconfigured to generate a resonant frequency.
 7. The optical transmittingsystem of claim 7, wherein the resonant frequency is approximately equalto a frequency of the modulation signal.
 8. The optical transmittingsystem of claim 1, wherein the switch is an n-type metal oxidesemiconductor (NMOS) transistor.
 9. A resonant illumination driver,comprising: a first inductor configured to receive a drive current; asecond inductor in a series with the first inductor; a capacitor inseries with the first and second inductors; and a power transistor inseries with the capacitor, the power transistor configured to switchbetween an on state and an off state to drive a light source.
 10. Theresonant illumination driver of claim 9, wherein the first and secondinductors are configured to generate a voltage at the capacitorsufficient to drive a gate of the power transistor to switch from theoff state to the on state.
 11. The resonant illumination driver of claim10, wherein: a first end of the first inductor is connected to a firstend of the second inductor and a second end of the first inductor isconnected to the capacitor; a second end of the second inductor isconnected to a source of the power transistor; and the capacitor isconnected to the gate of the power transistor.
 12. The resonantillumination driver of claim 11, wherein an inductance of the firstinductor is greater than an inductance of the second inductor.
 13. Theresonant illumination driver of claim 9 wherein a gate capacitance ofthe power transistor is greater than a capacitance of the capacitor. 14.The resonant illumination driver of claim 9, wherein the first inductorand second inductors and the capacitor are configured to generate aresonant frequency.
 15. The resonant illumination driver of claim 14,wherein the first and second inductors and the capacitor are configuredsuch that the resonant frequency is unaffected by a gate capacitance ofthe power transistor.
 16. A three dimensional (3D) time of flight (TOF)camera, comprising: a transmitter configured to generate an opticalwaveform with amplitude modulation corresponding with a frequency of agenerated modulation signal, the transmitter including an illuminationdriver configured to drive a light source that generates the opticalwaveform, the illumination driver including: a switch configured toswitch between an on state and an off state to drive the light source;and a switch driver configured to drive the switch between the on andoff states, the switch driver including a first inductor and a secondinductor in a split configuration with the first inductor and acapacitor in series with the first and second inductors and the switch;and a receiver configured to receive the optical waveform reflected offof a target object and determine a distance to the target object basedon a TOF from the transmitter to the target object and back to thereceiver.
 17. The 3D TOF camera of claim 16, wherein a capacitance ofthe capacitor is less than a gate capacitance of the switch.
 18. The 3DTOF camera of claim 16, wherein: a first end of the first inductor isconnected to a first end of the second inductor and a second end of thefirst inductor is connected to the capacitor; a second end of the secondinductor is connected to a source of the switch; the capacitor isconnected to a gate of the switch; and an inductance of the firstinductor is greater than an inductance of the second inductor.
 19. The3D TOF camera of claim 16, wherein the first inductor and secondinductors and the capacitor are configured to generate a resonantfrequency that is unaffected by a gate capacitance of the switch. 20.The 3D TOF camera of claim 19, wherein the resonant frequency isapproximately equal to the frequency of the modulation signal.